lib/aggregation_cursor.js

The AggregationCursor class is an internal class that embodies an aggregation cursor on MongoDB
allowing for iteration over the results returned from the underlying query. It supports
one by one document iteration, conversion to an array or can be iterated as a Node 4.X
or higher stream

lib/command_cursor.js

The CommandCursor class is an internal class that embodies a
generalized cursor based on a MongoDB command allowing for iteration over the
results returned. It supports one by one document iteration, conversion to an
array or can be iterated as a Node 0.10.X or higher stream

lib/cursor.js

The Cursor class is an internal class that embodies a cursor on MongoDB
allowing for iteration over the results returned from the underlying query. It supports
one by one document iteration, conversion to an array or can be iterated as a Node 4.X
or higher stream

lib/gridfs/grid_store.js

GridFS is a tool for MongoDB to store files to the database.
Because of the restrictions of the object size the database can hold, a
facility to split a file into several chunks is needed. The GridStore
class offers a simplified api to interact with files while managing the
chunks of split files behind the scenes. More information about GridFS can be
found here.